Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
Each gradient coil used in an MRI system, or other elements in the MRI system, may consist of a plurality of layers including conducting copper sheets or boards and insulation layers (e.g., an epoxy resin). FIG. 1 is an exemplary prior art laminate stack for a gradient coil. The gradient coil stack 100 includes a copper sheet or board 102, a laminate 106 (e.g., a fiberglass substrate) that is used as a backing for copper 102, and an insulation coating such as epoxy resin 104. The copper sheet 102 may be etched with a pattern or trace (e.g., a “fingerprint” pattern). Various processes, such as lamination or vacuum pressure impregnation (VPI), may be used during the manufacture or fabrication of the gradient coil 100. For example, in one fabrication process, the copper sheet 102 is laminated directly to the laminate 106. The entire board may then be impregnated with epoxy resin 104 using a VPI process. For example, the resin 104 may be injected into a mold (while the mold is under a vacuum) that forms the shape of the gradient coil.
During fabrication of the gradient coil, voids may form at an interface 120 between the copper 102 and the resin 104, at an interface 122 between the copper 102 and the laminate 106, in the resin 104, or in the laminate 106. For example, a void 108 is shown in FIG. 1 in the resin 104 at the interface between the copper 102 and resin 104. There are several possible sources of void formation in a gradient coil. One source of void formation is poor bonding/bonding strength between the copper 102 and the epoxy resin 104 or between the copper 102 and the laminate 106. For example, typically the epoxy resin does not bond well to an inorganic metal. Another source of void formation are leaks in the mold used for vacuum pressure impregnation of the epoxy resin 104. Leaks in the mold may form bubbles inside the VPI epoxy resin. In addition, incomplete coverage (e.g., delamination) during pressing of the copper 102 and laminate 106 is another possible source of voids.
Any voids that form in the gradient coil, e.g., void 108 shown in FIG. 1, are susceptible to partial discharges. An electric field is induced in a void and at a partial discharge inception voltage (PDIV), the voltage difference across the void causes a small spark (or partial discharge) to bridge the void. The spark or partial discharge causes a radio frequency (RF) noise burst to be emitted. The MRI system can detect the RF noise burst which will cause artifacts in the MRI image generated by the MRI system. For example, a partial discharge in a gradient coil may yield an effect in k-apace known as a “white pixel.” The “white pixel” produces an artifact in the reconstructed MR image making an image undesirable and difficult to interpret.
Accordingly, there is a need for a gradient coil and method of fabricating a gradient coil that reduces the number of voids and reduces or eliminates the partial discharge in voids that are formed in the gradient coil. In addition, it would be desirable to provide a gradient coil and method of fabricating a gradient coil that improves the bonding of the copper to an insulation layer and the bonding of the copper and to the fiberglass substrate. It would be also advantageous to provide a method of eliminating the field generation across a void in an MRI system by equalizing the potential difference across the void.